In the age of digital communication, needing to sign important documents online has become increasingly common, but it’s not necessarily the easiest process.
For an iPhone you can just use the standard Photos app to open it, hit Edit, click the Markup button, and add signature. Useful that you can also move it around.
I see on my Samsung Galaxy phone I can similarly use the Samsung Notes app and just a PDF with the S Pen. The selection tool will also allow me to select and move that signature around or resize it.
Both phones will allow you to sign, or complete, a document offline and no 3rd party apps needed.